This invention relates to the fabrication of piezoelectric devices which include crystal plates, and in particular to a method of contouring the crystal plates.
Piezoelectric devices, such as resonators and filters, rely on the frequency response of a crystal plate, which is typically quartz, to an electric bias supplied by means of electrodes formed on the surfaces of the plates. In many applications, it is desirable to contour the edges or major faces of the plates after defining the plate dimensions in order to achieve optimum frequency control. The need is especially acute for small plates, such as those used in oscillators and clocks for microprocessors, since it is difficult to otherwise confine vibration under the electrodes.
At the present time, contouring is usually done mechanically by either optical lens-type machining (e.g., grinding, lapping or polishing) of individual plates or by "pipe-contouring" where several plates are inserted, together with an abrasive, in a rotating pipe. The lens-type machining is precise but also very costly since each plate must be individually contoured. Pipe-contouring provides a batch-contouring process, but is not precise and is not particularly suited for contouring small and thin plates (diameters less than approximately 0.3 inches).
Recently, a batch-contouring method has been proposed which involves chemical etching of plates provided in a stack. (See U.S. patent application of A. J. Miller, Ser. No. 453,348, filed Dec. 27, 1982 and assigned to the present assignee, now U.S. Pat. No. 4,411,731, which is incorporated by reference herein.) This method appears most advantageous for contouring plates intended for operation above approximately 5 MHz (thicknesses typically below 0.012 inches). It would be desirable to provide an alternative batch-contouring method which is easily adapted for fabricating lower frequency devices which generally require a steeper contour.
It is, therefore, a primary object of the invention to provide a precision method of batch-contouring crystal plates.